Fundamentals of co-crystallization of polyoxacyclobutane and water

University Of Pittsburgh, Pittsburgh PA

Investigators

Abstract

Crystals composed of two distinct molecules organized within a lattice (co-crystals) are variously known as clathrates, host-guest compounds, intermetallic compounds amongst metals, or hydrates if water is included. A well-known example of an organic co-crystal is the gas hydrate formed from the co-crystallization of methane and water. The co-crystallization of polymers, very long chain organic molecules, with small molecules is unusual, and those with water are especially rare. The focus of this research project is to study a rare class of co-crystals based on the polymer, polyoxacyclobutane (POCB), and water. The researchers will investigate the types of phases that POCB hydrate can form as well as the rates of formation. Understanding the unique behavior of POCB hydrate is important, because it could be applied to a thermal desalination process that involves no membranes. The project will involve training graduate and undergraduate students in state-of-the-art research, as well as some curriculum development related to the topic of the project. The researchers will also perform outreach activities to pre-college students around the topic of molecular shapes and intermolecular interactions, especially in the context of gas hydrates and their relevance to energy. POCB can co-crystallize with water to form a hydrate, which melts upon heating above 37 degrees Celsius. Melting yields either a homogeneous liquid phase, or a water-rich and a polymer-rich two-phase mixture. The properties of the POCB hydrate are unique: (1) the melting point of the co-crystal exceeds that of the two pure species; (2) co-crystallization can occur either from a liquid-liquid equilibrium (LLE) state or from a homogeneous liquid solution; (3) the hydrate is highly insensitive to salt; and (4) the hydrate melting point fortuitously happens to be almost exactly at human body temperature. This combination of properties makes POCB unique not just amongst polymers, but amongst all materials. Very little is known about the POCB hydrate. Even the most basic information, such as the melting point of the hydrate in the high molecular weight limit or its heat of fusion, is missing. This research project will explore the thermodynamics and crystallization kinetics of POCB hydrate and examine the coupling between LLE and crystallization in which the polymer co-crystallizes with the solvent. The unique phase behavior of POCB could be used for a purely-thermal desalination process that requires no vacuum or pressure, no refrigeration, and no membrane. Such a process may enable waste-heat-based desalination and the desalination of high salinity waters. This research project will provide the first proof-of-principle and a preliminary technoeconomic evaluation of such a process. This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.